As a semiconductor light-emitting device, a superluminescent diode (SLD) has characteristics of emitting, while having a wide emission spectrum width that is relatively close to that of a light-emitting diode, light with a narrow radiation angle and high intensity as in a semiconductor laser emission state at the same time.
An SLD described in Patent Literature 1 includes a linear ridge waveguide formed to be vertical to a cleavage edge surface in plan view and a bending guide active layer provided so as to be bent subsequent to the linear ridge waveguide. An AR (antireflection) film may be formed on the cleavage edge surface. In the SLD having such a structure, a large portion of light generated in an active layer right below the linear ridge waveguide travels toward the bending guide active layer. The light traveling toward the bending guide active layer is separated into light that leaks due to the bend, light that is guided to an edge surface (edge surface on other side of cleavage edge surface) and reflected by that edge surface, and light absorbed while being guided. According to such a structure, the light that leaks due to the bend and the light that is reflected by the opposite edge of the cleavage edge surface cannot return to the linear active layer, so laser mode oscillation is suppressed (see, for example, lower right-hand column on second page to upper left-hand column on third page, and FIGS. 1A and 1B).
In short, instead of a structure of causing light to reciprocate by mirrors provided on both edge surfaces to cause oscillation like a normal laser diode (LD), the SLD includes a structure of causing light to travel unidirectionally in the waveguide and amplifying the light (stimulated emission is performed). A point that differs between them is that a wavelength spectrum width of output light of the SLD is far wider than that of the LD.
A superluminescent diode disclosed in Patent Literature 2 includes a core of a light emitting optical waveguide corresponding to an area where an electrode is formed and a core of a light absorbing optical waveguide capable of absorbing light, that corresponds to an area where an electrode is not formed. This light absorbing optical waveguide core becomes wider from a connection portion with the light emitting optical waveguide core toward a read edge surface. With such a configuration, light emitted rearward (direction toward opposite edge side of light-emitting edge) from the light emitting optical waveguide core is absorbed by the light absorbing optical waveguide core, with the result that return light of reflected light from the rear edge surface is suppressed. Therefore, low coherency is realized (see, for example, paragraph [0012] in specification).